The brick on my fireplace is badly...

Try applying a paste of cream of tartar and water. Rub it in, let it dry, and then scrub it off. Or create your own cleaner by following this recipe: Dissolve 1/2 cup naphtha soap in 1 quart hot water. Let cool. Stir in 1/2 cup ammonia and 1/2 pound powdered pumice. Apply the solution to your brick with a paintbrush, let it soak in, scrub, and rinse with warm water. Wear gloves to apply this, and avoid spilling the cleaning solution on floors or rugs. Make sure the room is well ventilated.